San Francisco city government will no longer be allowed to use city money to buy bottled water for its employees under an executive order Mayor Gavin Newsom is expected to sign today.

Despite owning a pristine reservoir in the Sierra Nevada that is said to produce some of the country's best-tasting tap water, the city spends nearly $500,000 a year on bottled water.

Newsom is making good on a year-old promise to curb spending on bottled water in the wake of a 2006 Chronicle story that found San Francisco had paid more than $2 million for water, paper cups and dispenser rentals in recent years.

"All of this waste and pollution is generated by a product that by objective standards is often inferior to the quality of San Francisco's pristine tap water," Newsom wrote in a two-page executive order that he is expected to sign today.

In addition to city departments, all city concessions, city-funded events and functions in city buildings will be prohibited from using city money to buy bottled water by July 1. By Dec. 1, all city departments located on city property must switch from bottled water dispensers to dispensers that attach to taps or water pipes and use water from the Hetch Hetchy Reservoir in Yosemite National Park.

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Those dispensers cost about $400 each, but the city's environmental director, Jared Blumenfeld, said that in the long run the cost will be cheaper than the $500,000-a-year bottled water bill the city currently pays.

"There's no rational reason for people to buy it," Blumenfeld said.

In 2005, Los Angeles Mayor Antonio Villaraigosa ordered city agencies to stop buying bottled water for employees after the media reported that the city had spent nearly $90,000 on it. At the same time, the city water agency was financing a $1 million ad campaign praising the virtues of what came out of the tap.

In San Francisco, taste tests have shown that the city's tap water tastes just as good as -- if not better -- than bottled water, but city agencies continue to spend money on bottled.

Even the city's Public Utilities Commission, which in 2005 performed a blind taste test on the street during National Drinking Water Week to convince people that San Francisco tap water is worth drinking, has purchased thousands of dollars worth of bottled water.

The Chronicle's report in 2006 found the departments that spent the most money on bottled water and related expenses during the previous year were Public Health, which spent $139,926; Muni, which spent $65,780; and San Francisco International Airport, which spent $65,670.

The mayor's office in City Hall spent $1,660, and the Public Utilities Commission, which provides drinking water to 2.4 million Bay Area customers, spent $8,622 on bottled water.

Gordon Bennett, a member of the executive committee of the San Francisco Bay chapter of the Sierra Club, said the plastic that bottled water comes in is not recyclable and takes up valuable landfill space.

"The other aspect is that there's nothing wrong with tap water," Bennett said. "In many cases, the quality coming out of the tap is equal to or better than bottled."